1. Field of the Invention
The present invention relates to a hearing aid having a switched amplifier.
2. Description of the Prior Art
Switched amplifiers, known also as class D-amplifiers have been employed in hearing aids such as for instance disclosed in U.S. Pat. No. 4,592,087. The purpose of the amplifier is to output the microphone signal to the earphone with a sufficiently high power.
Important considerations in a hearing aid application are integrateability, i.e., the amplifier should lend itself to be designed as a so called integrated circuit, and low power consumption. A switched amplifier meets these demands better than an amplifier which operates with permanently controlled transistors (see "Modulated Pulse Audio Power Amplifiers for integrated circuits" by H. R. Camenzind, IEEE-Trans. Vol. AU-14, Nr. 3, September 1966).
Switched amplifiers also transmit an alternating current U to the earphone when no audio signal is fed from the microphone. In such a case the signal U is without any modulation and consists only of an alternating voltage component of which the frequency f is high enough that it is not emitted from the earphone as an accoustic signal. Despite that, a so called holding current flows in the earphone, which is undesired and may in practical cases make up to 50% of the total holding current consumption of the hearing aid. The holding current amounts to: EQU I=U/Z.sub.f
In order to reduce I it is possible to increase the impedance Z.sub.f for the frequency f of the unmodulated output signal U by a low-pass filter, which is a common procedure for applications outside of the field of hearing aids but would occupy two much space in a hearing aid. A reduction of U does indeed reduce I, but limits the output power level which is attainable.